Investigating Possible Dipole‐Bound States of Cyanopolyynes: the Case for the C₅N⁻ Anion Detected in Interstellar Space

Abstract

We present quantum structure calculations aimed at demon‐ strating the possible existence of dipole‐bound states (DBS) for the anion C5N−, a species already detected in the Inter‐ stellar medium (ISM). The positive demonstration of DBS existence using ab initio studies is an important step to‐ ward elucidating possible pathways for the formation of the more tightly bound valence bound states (VBS) in envi‐ ronments where free electrons from starlight ionization pro‐ cesses are known to be available to interact with the radical partner of the title molecule. Our current calculations show that such DBS states can exist in C5N−, in agreement with what we had previously found for the smaller cyanopolyyne in the series: the C3N− anion. The predicted binding en‐ ergies which we found are 3 and 9 cm−1 for the 1Σ+ and 3Σ+ DBS, respectively. Furthermore, equilibrium geome‐ tries, equilibrium rotational constants, electric dipole mo‐ ments, and relative electronic energies were determined for the ground anion X1Σ+ and the two lowest electronic states of the parent neutral, X2Σ+ and A2Π.